Aluminum nitride which is obtained by nitriding aluminum completely is superior in thermal conductivity and electric insulating property so that it is used as a substrate material. This aluminum nitride is produced by carbon reduction of alumina or direct nitriding of an aluminum powder. In the direct nitriding methods of the aluminum powder, the following method is practically used in industrial field; the aluminum powder whose particle diameter is not more than 400 .mu.m is used, a process for nitriding is conducted at the temperature ranging from 900 to 1400.degree. C. and after that, crushing and refining thereof are conducted in the method.
Furthermore, in the direct nitriding methods of an aluminum powder, in order to improve the nitriding property thereof, the following methods are also known: a method in which the aluminum powder is crushed so as to be in a shape of a scale and a mixture of the obtained aluminum powder in the shape of scale and an aluminum nitride powder are used as raw materials; a method in which a metal-aluminum powder whose particle diameter is not more than 250 .mu.m is applied to nitriding once at a temperature which is not more than the melting point of aluminum, then the obtained product is crushed to have a mean particle diameter not more than 15 .mu.m and after that nitriding of them are conducted at a temperature ranging from 1300 to 1400.degree. C. (Japanese Unexamined Patent Publication (KOKAI) No. 61-83608); a method in which nitriding of a raw material in which a metal-aluminum powder is mixed with a fluorine included ammonium compound and an aluminum nitride powder is conducted at a temperature ranging from 430 to 650.degree. C. and after that, nitriding of the obtained product is conducted at a temperature ranging from 900 to 1300.degree. C. (Japanese Unexamined Patent Publication (KOKAI) No. 62-3007).
Also, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 5-279002, there is disclosed a method for producing an aluminum nitride powder whose crushing property is improved by conducting a process for nitriding in the following conditions: particle diameter of aluminum powder being used as a raw material ranges from 10 to 60 .mu.m, particle diameter of AlN being used as a nitriding accelerator agent which is mixed into ranges from 0.5 to 3 .mu.m and bulk density of the mixed product ranges from 0.6 to 1.9 g/cm.sup.3.
In Japanese Unexamined Patent Publication (KOKAI) No. 5-279002, the upper limit of particle diameter of an aluminum powder raw material which is applied to nitriding is set to be 60 .mu.m. This is because in the case if the particle diameter thereof exceeds 60 .mu.m, the enough nitriding reaction is not expected to be proceeded. Therefore, in Japanese Unexamined patent Publication (KOKAI) No. 5-279002, the aluminum powder whose particle diameter exceeds 60 .mu.m can not be used as a raw material, so that the above-mentioned publication has the disadvantage that it needs to use a fine aluminum powder which is relatively high in cost.
However, if a fine aluminum powder is used as a raw material, the aluminum powder as a raw material is clogged closely at the time of nitriding so that air permeability can not be conducted enough and nitriding reaction is hard to be generated. Accordingly, in order to conduct nitriding enough, temperature of nitriding is set to be high (for example, nitriding is processed under a temperature which is not less than the melting point of aluminum). As the result, after nitriding, aluminum powders strongly aggregate each other so that there arises a disadvantage that the obtained product is not easy to be crushed.
In order to solve the above-mentioned disadvantage, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 5-279002, by using an ALN powder whose particle diameter is finer compared with the ordinary particle diameter thereof (4 to 10 .mu.m) and which has the effect to prevent sintering, aluminum powders are properly isolated each other so as to prevent aggregation at the time of nitriding. However, it is necessary to mix a relatively large amount of the ALN powder which is used so that leads to be high in cost and its productivity is inferior. Furthermore, it is necessary to control the bulk density of the mixed product.
The object of the present invention is to solve the problems described above.